Unexamined Japanese Patent Application KOKAI Publication No. 2007-187581 discloses a distance measuring apparatus that includes a cylindrical convex lens where flat beam enters, a camera which picks up a laser spot (hereinafter, referred to as a projected light spot) that is projected on an object by the beam passed through the cylindrical convex lens, and a distance obtaining unit which obtains a distance to the object based on the shape of the projected light spot picked up by the camera. The distance measuring apparatus allows flat beam to enter the cylindrical convex lens with a flat plane tilted by 45 degrees relative to the axial direction of the cylindrical convex lens, so that the beam (hereinafter, referred to as twisted beam) emitted from the cylindrical convex lens changes a rotation angle of the flat plane in accordance with the distance from the cylindrical convex lens. Accordingly, the shape of the projected light spot formed on the object becomes the shape of a line segment which changes a rotation angle in accordance with the distance from the cylindrical convex lens. Therefore, the distance measuring apparatus disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2007-187581 detects a rotation angle of the projected light spot picked up by the camera and measures a distance to the object based on the detected rotation angle.
It is difficult to reduce the size of the cylindrical lens, so that miniaturization of the distance measuring apparatus disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2007-187581 is difficult.
The present invention has been made in view of the above-explained circumstance, and it is an object of the present invention to provide a downsized diffractive optical element that emits twisted beam, a downsized distance measuring apparatus and a distance measuring method using the downsized diffractive optical element.